In case this helps anyone.
The main story disappeared after finishing the oracle quest but after installing the update of the game (on PS4) I pressed continue (which had hours of game time after the main quest disappeared) and right away I got the 'fanfare notification' that I had completed the oracle quest (search for the truth I think it was called) even though I finished it hours ago, and I could now find Herodotus and continue to the next quest! Still though, there was no odyssey quest visible in the menu. I finished that one and moved to the next where you are supposed to meet Herodotus again at another location. He was there but I was unable to talk to him and still no sign of an odyssey quest in the menu (meaning I found him by remembering where he said to meet, since I couldn't track the quest), so I quit the game and opened it again and voilą - Herodotus was again approachable and the odyssey quest was back!

ItsYaBoyCynical

10-10-2018, 11:18 PM

Please let us know if, after completing that specific chain of the main quest, it disappears again. Some people have mentioned that, I'm on PC so I can't test until tomorrow. If the bug is just gonna immediately disappear again I won't even bother.